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At-grade Intersection Speed-Change Lanes
Conflicts are created along highways wherever driveways, intersections and other access points are introduced. These conflicts involve traffic traveling on the highway, crossing traffic, and traffic turning into or out of an intersecting roadway.
To reduce conflicts at these access points, right-turn and left-turn speed-change lanes are used. Speed-change lanes provide a separate facility for turning vehicles to decelerate or accelerate, and to queue while waiting to turn.
As a result, speed-change lanes minimize the interference between through traffic, crossing traffic, and turning traffic along the highway, creating a safe environment for the traveling public.
At rural unsignalized at-grade intersections, four types of speed-change lanes are generally used in Iowa: (1) left-turn median deceleration lane; (2) right-turn deceleration lane; (3) left-turn median acceleration lanes; and (4) right-turn acceleration lanes.
Type 1 - Left-turn median deceleration lane – Commonly used in Iowa, a left-turn median deceleration lane is located in the median and left of the mainline traffic that lets a vehicle decrease its speed, stop to wait for a gap in traffic, and then cross the traffic lanes to enter the intersecting roadway.
Type 2 - Right-turn deceleration lane - Right-turn deceleration lanes reduce the risk of rear-end collisions by allowing speed reduction and right turns to take place outside of the highway through lanes. Depending on their length, either full or partial speed reduction can occur within the deceleration lane, thereby reducing speed differentials and associated traffic disruption in the through lanes.
Type 3 - Left-turn median acceleration lane (MAL) – A lane located in the median and left of the mainline traffic lanes that lets a vehicle increase its speed and safely merge with traffic.
Left-turn MALs were originally constructed in locations with high truck traffic to allow trucks a safer and easier opportunity at merging with mainline traffic. However, recently they have been looked at as a way to increase intersection safety and decrease delay time for all vehicle types. They are designed to provide benefits for vehicles making a left turning maneuver from a minor two-lane road onto a four-lane divided highway. The theory behind a left-turn MAL is that they allow left-turning vehicles to make the turn once there is a gap in the lanes on the near side of the median instead of a gap in the passing lane on the far side of the median and in the lanes on the near side of the median, without stopping in the median. Once vehicles cross the nearest lanes they can immediately make the left turn and begin to accelerate. This should allow vehicles to merge at higher speeds and reduce delay time at intersections. Since the speed differential between the merging traffic and the mainline traffic is reduced, left-turn MALs should reduce the number of accidents near the intersections. Rear-end and sideswipe crashes should be reduced as a result of the higher merging speeds.
Type 4 - Right-turn acceleration lane – A right-turn acceleration lane allows a vehicle entering a roadway to increase its speed to a rate that it can merge with through traffic, and gives the main roadway traffic time and distance to make appropriate adjustments to allow for the vehicle to safely merge.
When are speed-change lanes warranted?
Warrants for the use of speed-change lanes cannot be stated definitely. Many factors must be considered, such as speeds, traffic volumes, percentage of trucks, highway geometry, frequency of intersections, and crash experience.
Deceleration lanes are advantageous on high speed roads because the driver of a vehicle leaving the highway must slow down in a through-traffic lane if a deceleration lane is not provided. The failure to brake by the following drivers because of a lack of alertness causes many rear-end collisions.
Acceleration lanes are not always cost-effective at stop-controlled intersections where entering drivers can wait for an opportunity to merge without disrupting through traffic. Acceleration lanes can be advantageous on roads without stop control and on all high-volume roads even with stop control where openings between through vehicles in the peak-hour traffic streams are infrequent and short.
How to safely use speed-change lanes
Step 1. | When exiting the highway to the left, use the left-turn deceleration lane. This lane allows you to slow down before reaching the exit, without being a hazard to other vehicles on the highway. |
Step 2. |
Turn on your left-turn signal. |
Step 3. |
Begin slowing down before you enter the deceleration lane and come to a complete stop in the median area. |
Step 4. |
Scan the traffic flow in the opposing lanes of traffic and look for a safe break. |
Step 5. |
Make the left turn by crossing the opposing lanes of traffic and entering the intersecting roadway. |
Step 6. |
Turn off your left-turn signal. |
Step 1. | When exiting the highway to the right, use the right-turn deceleration lane. This lane allows you to slow down before reaching the sideroad without being a hazard to other vehicles on the highway. |
Step 2. |
Turn on your right-turn signal. |
Step 3. |
Begin slowing down before you enter the deceleration lane to reach a speed that allows for a safe right turn. |
Step 4. |
Make the right turn. |
Step 5. |
Turn off your right-turn signal. |
Step 1. | Turn on your left-turn signal. | |
Step 2. |
Start your left-turn maneuver onto the highway by first looking for a break in traffic from your stopped position at the roadway intersection. |
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Step 3. |
When a break is available, cross the first traffic lanes and enter the median storage area. |
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Step 4. |
Take a quick look to the right, left and right again to make sure you can safely enter the acceleration lane. Be sure to check for possible hazards or vehicles in the lane. |
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Step 5. |
If safe, immediately move into the acceleration lane and begin increasing your speed. |
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Step 6. |
Reach a speed that will allow you to move right safely into the mainstream traffic. |
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| Step 7. | Check your mirrors, check your blind spots, and check for a break in the mainstream traffic flow. Blind spots are areas slightly to the sides and the rear of your vehicle that you can not see in your rear view or side view mirrors.
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Step 8. |
If safe, change lanes by merging right into the adjacent mainline traffic lane. |
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Step 9. |
Turn off your left-turn signal. |
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Do not stop in the acceleration lane unless absolutely necessary. Remember, you must yield the right of way to the traffic already on the highway.
Right-turn acceleration lane
Step 1. | Turn your right-turn signal on. | |
Step 2. |
Begin your right-turn maneuver by positioning your vehicle behind the white line and yield sign. Scan the mainline traffic flow to make sure you can safely enter the acceleration lane. Be sure to check for possible hazards or vehicles in the lane. |
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Step 3. |
If safe, immediately move into the acceleration lane and begin increasing your speed. |
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Step 4. |
Reach a speed that will allow you to move safely left into the mainstream traffic. |
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Step 5. |
Check your mirrors, check your blind spots, and check for a break in the traffic flow. Blind spots are areas slightly to the sides and the rear of your vehicle that you can not see in your rear view or side view mirrors.
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Step 8. |
If it is safe, change lanes by merging into the adjacent mainline traffic lane. |
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Step 9. |
Turn off your signal after completing the lane change. |
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Do not stop in the acceleration lane unless absolutely necessary. Remember, you must yield the right of way to the traffic already on the highway. Be aware of the blind spots.
Highway Safety Rules
Parking, stopping, loading or unloading
A vehicle may not be stopped, left standing, parked, loaded or unloaded on an acceleration or deceleration lane, or on the adjacent shoulder.
Emergencies and right-turn acceleration and deceleration lanes
In case of emergency only, parking, stopping, standing, loading or unloading of a vehicle is permitted on the shoulder to the right of the travel lanes facing with the direction of travel when the wheels of the vehicle and the projecting parts of the body or load are safely off and to the right of the travel lanes.
Emergencies and left-turn acceleration and deceleration lanes
In case of emergency only, parking, stopping, standing, loading or unloading of a vehicle is permitted on the shoulder to the left of the travel lanes facing with the direction of travel when the wheels of the vehicle and the projecting parts of the body or load are safely off and to the left of the travel lanes.
Passing or general travel
Motor vehicle operators should NEVER use the acceleration and deceleration lanes as general travel lanes or for passing other vehicles.
Allow vehicles to safely merge
Be courteous to vehicles entering the highway and allow them to merge.
If you observe a vehicle using the left-turn median acceleration lane, move to the right lane, if you can do so safely and without slowing the traffic flow, to allow the vehicle to safely merge into the mainline of traffic.
If you observe a vehicle using the right-turn acceleration lane, move to the left lane, if you can do so safely and without slowing the traffic flow, to allow the vehicle to safely merge into the mainline of traffic.
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